It is well known in the sewing machine art that, in order for the lockstitch sewing machine needle to effectively stitch pieces of fabric together and to make complete stitches, a bottom stitch be available to interact with the needle thread which secures the needle thread in the fabric. The thread which is essential for this purpose is located on a bobbin which during the sewing cycle or sewing process of the sewing machine is located physically below the throat plate of the sewing machine and is positioned such that the needle thread will be picked up by and intertwined with the bobbin thread to secure the stitches in the fabric.
If the bobbin, for one reason or another, becomes empty there will, of course, not be a bottom thread with which the needle or sewing thread can be intertwined and thus the stitches made during those periods when the bobbin thread is not present will not be held in the fabric and the piece of fabric or material being sewn will have to be re-sewn.
Therefore, it would be desirable to be able to monitor the condition of the bobbin with respect to the amount of bobbin thread remaining and to provide a signal to the sewing machine operator that the bobbin was empty or that there was a predetermined amount of bobbin thread remaining on the bobbin and that a certain preselected number of additional stitches could be correctly produced with the amount of bobbin thread which remains on the bobbin. This is, of course, desirable in any commercial sewing operation, but it would be likewise desirable to have this indication present in non-commercial uses.
The ability to inform the operator that a certain amount of material or thread remains on the bobbin is of critical importance in those sewing operations where predetermined lengths are being sewn on an intermittent basis with a certain amount of bobbin thread being required for each stitch cycle. If the amount of bobbin thread remaining on the bobbin could be determined, the operator could be informed when there was a sufficient amount of material to perform two or three more sew cycles. In this way the operator would be given some leeway as to when the bobbin would have to be replaced and yet she would not have gotten to the point of having run out of bobbin thread which would require some amount of re-sewing.
The present invention provides a device and method for determining when a bobbin has run out of thread and additionally for determining when a specific amount of bobbin thread remains on the bobbin. The device is coupled to the foot switch operated by an operator of the sewing machine so that when the foot switch is raised, de-energizing the sewing machine motor and thus stopping all sewing activity, a probe is inserted in the side wall of the bobbin housing and the bobbin spool is itself grounded. If the probe touches the hub of the bobbin spool, a circuit will be completed through a signalling device which will then indicate to the operator that the bobbin has run out of bobbin thread and that a new bobbin needs to be provided. The probe device can also be provided with a back pressure sensing switch which can be adjusted along the length of the probe shaft and preset so that the back pressure switch is actuated when the needle reaches a preset depth within the bobbin housing. If the needle stops before reaching the preset depth, the pressure switch will not be tripped which will not make any indication that the bobbin is running low on the amount of thread remaining thereon. If the probe does, however, reach at least the preset point, the pressure switch will be closed and indicate that only a certain amount of bobbin thread remains on the bobbin from which the operator can then determine whether additional sewing cycles can be made or whether a new bobbin needs to be placed in the machine.